This invention relates to concrete foundations set within excavations or bore holes which are installed to support wind turbines. More particularly, this invention comprises an apparatus and method for configuring, installing, and setting the anchor bolts for a cylindrical foundation for a wind turbine prior to pouring the concrete.
U.S. Pat. Nos. 5,586,417 and 5,826,387, both by Henderson, disclose a foundation “which can be poured-on-site monolithically and is of cylindrical construction with many post-tensioned anchor bolts which maintain the poured portion of the foundation under heavy compression, even during periods when the foundation may be subject to high overturning moment.” Henderson's foundation is preferably in the shape of a cylinder, having an outer boundary shell and an inner boundary shell each formed of corrugated metal pipe. Between the outer boundary shell and the inner boundary shell elongated high strength steel bolts extend vertically up through concrete from a peripheral anchor plate, called an inbed plate, located near the bottom of the cylinder. The bolts extend upwardly from the inbed plate to a connecting plate or flange above the ground surface. The bolts extend through hollow tubes to prevent adhesion of the concrete to the bolts. The foundation typically uses no rebar reinforcing steel. This design uses the mechanical interaction with the earth to prevent over turning instead of the mass of the foundation typically used by other foundations for tower structures. FIG. 1 schematically shows an embodiment of the Henderson foundation.
The “hollow tubes” of this foundation are typically elongated plastic tubes which encase the bolts substantially through the entire vertical extent of the concrete and allow the bolts to be tensioned after the concrete has hardened and cured, thereby post-tensioning the entire concrete foundation. Alternatively, the elongated bolts can be wrapped in plastic tape, or coated with a suitable lubrication, which will allow the bolts to stretch under tension over the entire operating length of the bolt through the vertical extent of the concrete.
Henderson further discloses the post-stressing of the concrete in great compression by tightening the high strength bolts to provide heavy tension between a heavy top flange and the inbed plate at the bottom of the foundation, thereby placing the entire foundation under high unit compression loading. The bolts are tightened so as to exceed the maximum expected overturning force of the turbine tower on the foundation. Therefore, the entire foundation withstands various loads with the concrete always in compression and the bolts always in static tension.
The tensioning bolts in the cylindrical foundation are preferably in side-by-side pairs, the pairs extending radially from the center of the foundation, forming an inner ring of bolts and an outer ring of bolts as shown in FIG. 2. As shown in FIG. 2, the inner ring of bolts has a slightly shorter diameter than the outer ring of bolts. The bolt pattern is, of course, determined by the bolt pattern on the mounting flange of the turbine tower to be installed on the foundation. A large number of bolts in typically used for this type of foundation. Typically seventy tensioning bolts are used in the inner ring and seventy tensioning bolts in the outer ring for a total of one hundred forty. In Henderson's foundation, the lower ends of the bolts are anchored to the inbed plate at the bottom of the foundation which may be constructed of several circumferentially butted and joined sections.
The following known procedure is typically followed in constructing the cylindrical foundation. A bore hole is drilled or excavated and an outer boundary shell of corrugated metal pipe (“CMP”) is set within the hole. Bolt bundles are lowered into the borehole. The bolt bundles typically comprise about thirty bolts, with each bolt weighing up to two hundred pounds per bolt. Workers are lowered into the CMP lined bore hole. Working from the bottom of the bore hole, the workers lift and/or position each individual bolt so it can be inserted into a template at the surface, which is suspended above the bore hole by a crane having a capacity of approximately 100 tons. Once each bolt is inserted into the template, a nut made up onto the threads extending above the template, such that the weight of each bolt is suspended by the template.
Once all of the bolts have been suspended from the template, the entire assembly is lifted out of the bore hole so the inbed plates may be placed at the bottom end of the bolts. As the assembly is lowered back into the bore hole, bands or rebar wraps are placed around the collective bolts to hold the bolts in position during the pouring of the concrete. FIG. 3 shows such an assembly suspended by a lifting frame which is connected to the template. The entire assembly is then lowered back into the bore hole and an inner boundary shell of CMP is lowered into the bore hole such that the bolts are extending upwardly through the annulus formed by the outer boundary shell and the inner boundary shell. Concrete is poured into this annulus around the upwardly extending bolts, with the template at the top of the bolts used to form a “grout trough” in the upper surface of the concrete. The upwardly facing ends of the bolts extend into the grout trough and, following the hardening of the concrete, the grout trough is filled with a high strength grout upon which the tower flange is placed when the grout has adequately cured.
It is to be appreciated that the above-described procedure anticipates that a number of workers will be working below a very heavy assembly (i.e., the template with all of the suspended bolts) as shown in FIG. 3. After all of the bolts are hung from the template, the entire assembly, including the lifting frame, is very heavy, requiring the use of a crane having a lifting capacity of approximately 100 tons. This procedure has a number of disadvantages. For example, it requires a large number of workers working below a very heavy assembly, creating a potential safety hazard. It also requires a large number of personnel in the borehole to expeditiously hang the bolts from the template. A comparable number of personnel are required at the surface to install and tighten the nuts on the upwardly facing ends of the bolts. A large crane is required for lifting and suspending the bolt-template assembly.